A microwave frequency tube comprises a microwave frequency structure that is passed through by an electron beam generated by an electron gun. The electron beam is contained in a space where the interaction occurs between the electrons of the beam and an electromagnetic wave (traveling or standing), the field configuration of which wave is determined by the microwave frequency structure of the tube: resonant cavities in the case of the klystron and a delay line in the case of a traveling wave tube (TWT).
In most microwave frequency electronic tubes, a magnetic field is used to contain the beam in the interaction space for interaction with the microwave frequency wave. The tubes that are most widely used, like traveling wave tubes (TWT) and klystrons, use an electron beam of cylindrical geometry, which requires a magnetic field parallel to the axis of the electron beam.
The beam-containing magnetic field can be generated by a solenoid, or using permanent magnets around the microwave frequency structure of the tube. The use of permanent magnets eliminates the need for an electrical power supply for the solenoid, but requires a large-volume (and therefore very heavy) permanent magnet to generate a magnetic field with a single alternation in the interaction space. The term “alternation” should be understood to mean a determined direction of the beam-containing magnetic field.
To reduce the volume and the weight of the permanent magnet, an alternating magnetic field is used, generated by a series of permanent magnets, along the containment axis of the beam. The magnets provide alternate fields of opposite directions from one magnet to the next in the microwave frequency structure of the tube; the expression “periodic permanent magnet focusing”, with the acronym PPM, then applies.
This type of containment of the electron beam by alternating magnetic field is commonly used in traveling wave tubes (TWT) and in some klystrons. Since klystrons are tubes that are shorter than TWTs, the containment field comprises fewer alternations (single reversal permanent magnet: for two alternations; double reversal permanent magnet: for three alternations).
FIG. 1 shows a partial cross-sectional view of a microwave frequency structure of a helix TWT of the prior art.
The microwave frequency structure of FIG. 1, of circular cylindrical shape on an axis ZZ′ of propagation of a cylindrical electron beam 10, comprises a sheath 14 with incorporated polar shoes containing the helix 16 of the TWT. The sheath 14 is used both to mechanically secure the helix 16 in the microwave frequency structure via insulating supports 18, and to seal the tube.
The sheath 14 comprises an assembly of a series of pole pieces (or parts) 20 made of iron and non-magnetic spacers 22, a spacer separating two consecutive pole pieces forming spaces 24 incorporating toroid-shaped permanent magnets 30 generating the magnetic field for containing the electron beam on the propagation axis ZZ′.
The toroid-shaped magnets 30, with axes of revolution colinear to the axis ZZ′, and with rectangular sections, are magnetized parallel to the axis ZZ′. The direction of magnetization changes alternately from one magnet to another next or preceding magnet along the axis ZZ′, which produces a sinusoidal variation of the containing magnetic field generated by the magnets 30 along the axis ZZ′.
FIG. 2 shows a partial cross-sectional view of a section of the sheath 14 of the structure of FIG. 1.
The cross-sectional view of FIG. 2, along a plane of symmetry Ps passing through the axis ZZ′, shows the path of the magnetic flux lines Ch from the magnet 30 over a length corresponding to an alternation of the magnetic field (or half a period corresponding to two consecutive changes, on the axis ZZ′, of the direction of the magnetic field). The pole pieces 20 guide the magnetic flux generated by the permanent magnets to obtain a beam-containing magnetic field parallel to the axis ZZ′.
FIG. 3 shows a partial cross-sectional view of a microwave frequency structure of a coupled-cavity TWT 40.
As in the cases of the TWT of FIG. 1, polar shoes 42 guide the magnetic flux produced by the permanent magnets toward the axis ZZ′, which makes it possible to obtain a magnetic field parallel to the axis ZZ′, but, unlike the helix TWT of FIG. 1, the polar shoes have a second function: they form the walls of the successive cavities 40 forming the delay line of the tube.
The toroid-shaped permanent magnets 44 similar to those used in the helix TWTs are placed around the cavities 40; because of this, they have a larger diameter than those used on helix TWTs. On this type of coupled-cavity tube, the variation of the magnetic field, along the containment axis ZZ′, is not sinusoidal. In practice, one alternation contains two magnetic field peaks instead of one. The term concentrator with harmonic 3 then applies. This result is obtained by placing a magnetic core 46 or a polar shoe mid-way between the two polar shoes 42 which guide the magnetic flux either side of the permanent magnet 44. This type of concentrator is also suitable for klystrons comprising several single or multiple cavities (extended-interaction klystrons).
These devices for containing the beam in the microwave frequency structure of the TWTs, helix or coupled-cavity, have drawbacks.
For example, when a TWT is operating, a portion of the microwave frequency power propagated in the microwave frequency structure of the tube is lost in the form of heat. These losses take place on the helix or in the walls of the cavities depending on the TWT type (losses through skin effect) or in the helix-supporting dielectrics, in the lossy dielectrics used for matching (severe loads) or for absorbing parasitic modes (resonant buttons).
Moreover, a portion of the electrons from the beam is intercepted by the helix of the TWT or by the drift tubes between the cavities of a coupled-cavity TWT or a klystron. Thus, when an electron from the beam falls on the delay line formed by the helix of a TWT or on a klystron drift tube, its kinetic energy is converted into heat.
These two mechanisms, microwave frequency losses and electron beam interception, create a power flux at the core of the microwave frequency structure, which determines a maximum operating temperature of the structure according to the temperature of the cooling source surrounding the microwave frequency structure and of the thermal impedance between a central portion of the structure and the cooling source.
In the case of the helix TWT with microwave frequency structure represented in FIG. 1, the power dissipated as heat passes through the helix 16 toward a cold source outside the tube via the dielectric supports 18, the polar shoes 20, cooling fins of the tube then the tube cladding parts (not represented in FIG. 1).
In the case of a coupled-cavity TWT or the klystron, the dissipated thermal power passes through the drift tubes toward the cold source via the polar shoes, fins then the tube cladding parts.
In addition to the production of the magnetic field for containing the electron beam, the beam containing system of the microwave frequency tubes of the prior art is therefore used to cool the tube, which has drawbacks. In practice, the thermal conductivity of the iron of the polar masses is not as good as that of copper (80 W/m.K for iron and 398 W/m.K for copper). In both cases, the weak point of these structures is the cooling.
One solution for enhancing the cooling and increasing the average power delivered by a microwave frequency tube of the prior art consists in producing cooling channels between the hot internal portion of the microwave frequency structure and the permanent magnets. For example, on a coupled-cavity TWT (FIG. 3), the internal and external diameters of the magnets 44 can be increased to place the cooling channels between the outer diameter of the cavities 40 and the magnets.
However, this cooling solution is reflected in an increase in the volume and the weight of the microwave frequency structure and notably of the electron beam-containing device, which is not always compatible with the application considered.
Another solution for enhancing the cooling without excessively modifying the volume and the weight of the microwave frequency structure involves placing a thermal shunt between the central portion of the microwave frequency structure and the cold source to reduce the thermal impedance. To this end, the iron of the polar shoes can be replaced by copper and it is then more advantageous to use permanent magnets with a magnetic polarization in a plane perpendicular to the axis ZZ′ of the beam rather than parallel to the axis of the beam, since there are no longer any polar shoes to channel the flux lines toward the axis ZZ′. It is also possible to remove a portion of the volume of the magnet and replace it with copper to produce the thermal shunt.
FIG. 4a shows a partial view of the sheath 14 showing the field lines of the permanent magnet 30 of FIG. 2. The toroid-shaped magnet 30 between two polar shoes 20, magnetized parallel to the axis ZZ′ according to the arrow Fc (designated “axial magnetization”) is a conventional structure of the prior art.
FIG. 4b shows a partial view of another structure comprising two toroid-shaped permanent magnets A1, A2 magnetized along axes perpendicular to the axis ZZ′ (arrows Fc1, Fc2 in the figure) (designated “radial magnetization”) according to the solution involving replacing the polar shoes with copper.
FIGS. 4a and 4b show the plots of the flux lines Ch over a distance corresponding to a half-period following the axis ZZ′.
FIGS. 4a and 4b show that, for magnetized toroidal cores (or rings) with the same internal r and external R radii, the flux lines Ch are less close to the axis ZZ′ in the structure without polar shoes of FIG. 4b than in the conventional structure with polar shoe of FIG. 4a. Consequently, the intensity of the magnetic field on the axis ZZ′ created by the structure with permanent magnets with radial fields of FIG. 4b is weaker.
To mitigate the defect in the structure comprising permanent magnets of FIG. 4b with a radial magnetization, a toroid-shaped magnetic core 50 with external radius equal to the internal radius r of the permanent magnet 30 can be placed inside the permanent magnet 30, which effectively makes it possible to increase the intensity of the magnetic field on the axis ZZ′.
FIG. 5 shows the field lines of two contiguous magnets of the structure of FIG. 4b comprising two magnetic cores 50, the internal radius of which is equal to the internal radius of the pole pieces 20 of FIG. 4a. 
The structure with radially-magnetized permanent magnets, as represented in FIGS. 4b and 5, has another defect. In practice, the magnetic polarization of the magnets A1, A2 does not remain in a transverse plane, the magnetic lines in one of the magnets turn toward the neighboring magnet when the internal r and external R radii of the magnets are approached.
The magnetic flux that crosses the surface Sr based on the internal radius r represents only a fraction of the total flux created by the magnet, which results in a magnetic field in the axis ZZ′ that is weak. In order to oppose the flux passing through the lateral faces of the magnets, another permanent magnet, ring-shaped and magnetized axially (or parallel to the axis ZZ′) can be placed between two radially-magnetized permanent magnets.
FIG. 6 shows a variant of the structure of FIG. 4b. 
The structure of FIG. 6 comprises the two radially-magnetized toroidal permanent magnets A1, A2 separated by a third, axially-magnetized, ring-shaped permanent magnet A3.
The structure with three toroidal magnets A1, A2, A3 of FIG. 6 results in an increase in the peak field in the axis ZZ′. This increase in the magnetic field in the axis ZZ′ is confirmed by simulation calculations.
The topology of the structure of FIG. 6 with three toroidal magnets is equivalent to that of a conventional beam-containing device with axially-magnetized rings and polar shoes as represented in FIGS. 1 and 2 in which the polar shoes would have been replaced by radially-magnetized rings.
Apart from the magnetic cores, the use of radially- and axially-magnetized rings to produce a PPM concentrator having symmetry of revolution is known (see for example H. A. Leupold et al., Iron-free permanent magnet structure for travelling wave tubes, IEDM 1991, pp 411-414).
The structures with radially-magnetized permanent magnets include another drawback in their implementation because of the difficulty in producing magnetized rings with magnetization at all radial points. An approximation can be produced by gluing a number of radially-magnetized segments to form a complete toroidal core, but the result is not as good.
FIG. 7 represents various curves f variations of magnetic fields CMg calculated for different containing magnets, expressed in gauss as a function of the external radius R of the magnetized ring. The calculations are made for axially- or radially-magnetized rings of different thicknesses Ep, of the same internal radius r=3.2 mm, and of variable external radius R.
In FIG. 7, the curve Ref represents the magnetic field in the axis ZZ′ created by a ring of the prior art that is axially magnetized, the curves Ep4.2 and Ep3.2 show the magnetic fields created by radially-magnetized rings of respective thicknesses Ep=4.2 mm and Ep 3.2 mm. The point in FIG. 7 marked Ep4.2+N represents the magnetic field created by the 4.2 mm thick ring with toroidal core and the point marked Ep3.2+N represents the magnetic field created by the 3.2 mm thick ring with toroidal core.
In summary, the structure with radial (or radially-magnetized) magnets without polar shoes has three defects:                A magnetic field on the axis ZZ′ that is weaker than that produced by a conventional containing device because of the absence of polar shoes to guide the flux as close as possible to the axis ZZ′. This defect can be partially compensated by introducing magnetic cores under the magnets.        Flux lines Ch that are incurved to pass from one magnet to the next instead of remaining in a transverse plane and leaving through the surface corresponding to the internal diameter of the magnet. This defect can be partially compensated by the introduction of an axially-magnetized ring A3 between two radially-magnetized rings.        Finally, a production problem: rings with radial magnetization cannot be produced. Sections (or segments) of rings must be assembled and then glued together.        
Furthermore, to house a thermal sunt, for example a piece of copper replacing the iron of the polar shoes, the volume of the toroidal magnet must be reduced, which reduces the field on the beam containment axis ZZ′.